1. Field of the Invention
The present invention concerns an electric motor the "coreless" type.
2. The Prior Art
An electric motor of this kind is often a rotary (or circular) device but may equally well be a linear device.
Where a motor of this kind is used in space, it is often associated with one or more magnetic bearings.
In practice attempts are made to minimise the energy consumption of magnetic bearings, which entails reducing disturbances as much as possible, in particular disturbances caused by the associated motors. To this end "coreless" motors are preferred over "cored" motors, the major disadvantage of which is their radial stiffness.
A classic but non-limiting application of these "coreless" motors concerns the magnetic bearings of satellite wheels, where the environment is severe in terms of vacuum and the permissible temperature range.
Motors of this kind are described, for example, in U.S. Pat. Nos. 3.955.858 and 4.211.452.
These three-phase rotary type motors incorporate a rotor mobile in rotation relative to a stator. The stator carries a cylindrical exciter strip coaxial with the rotor formed by a threefold plurality of alternating coil windings connected in series within each plurality and circumferentially aligned with each other. These pluralities of windings, each corresponding to one electrical phase, are connected each to a respective current source through a switch adapted to pass current through them in an alternating manner. The exciter strip is disposed in an annular airgap along a ring of permanent magnets magnetised in alternate radial directions with their magnetic fields closed by magnetic closure rings disposed on each radial side of the airgap and the ring of permanent magnets.
The currents flowing in the pluralities of windings interact with the magnetic fields of the magnets to generate a motor torque defined by LAPLACE's law.
In practice the permanent magnets are offset relative to each other in the circumferential direction in order to minimise the EDDY currents (which are very high in large-diameter motors because of the high rotor/stator relative speeds), which results in a non-negligible fluctuation in the motor torque because of switching between successive electrical phases.